Ah, Lisa, I feel so bad for you! The misery of working two jobs while you're away and, then, being held hostage by this woman when you're home is an EMERGENCY. You're drowning, and let's stop thinking about whether lowering the lifeboat is legal. Save yourself now, worry about that shit later.
1) Line up a place to live for next month or asap.
2) Notify landlord that you are breaking the lease, and will not paying as of next month. Also, apprise him that you had a friend helping you with the rent, but she stopped contributing, and now she won't move out. Apologize for this, then tell him you have no resources, and cannot continue carrying the apartment, so GOODBYE.
3) Don't discuss with your friends, don't tell your roommate, just go. Hire movers with packing & boxes with your EF or credit cards, so you can get out all at once, and with other people present.
Once you're in the glorious peace of your own apartment, you can deal with whatever fallout might occur. Will she sue you? She might, but probably not. Suing is a hassle, and she barely leaves her room. It requires meetings, paperwork, jumping through hoops... very hard for someone with depression. She might need a lawyer to do it, but she doesn't have any money to pay one. And, how is she going to get someone else in her life to pay for a lawyer? There's nothing in it for the boyfriend: just one poor person suing another poor person. How much money could she even say she was out? At most, half the rent for the year? So, in that case, you'll negotiate that amount down, and then you'll negotiate a payment plan. It'll likely be for less than you're paying for her now!
It's even less likely that your landlord will sue you, but even if it happens, again, it won't be for some astronomical sum. You'll negotiate, you'll get on a payment plan, whatever.
It's 90% certain you could ghost the whole situation tomorrow and have no bad consequences whatsoever. In fact, you'd have more money in your pocket, and one less deadbeat roommate dragging you down.